This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
As is known to those of ordinary skill in the art, a virus is an infectious agent that replicates inside the living cells of organisms. Viruses infect all types of organisms, including bacteria, plants, and animals. Examples of common human diseases caused by viruses include the common cold, influenza, chickenpox, and cold sores. Many serious diseases such as Ebola, AIDS, avian influenza, and SARS are caused by viruses. Since the initial discovery of tobacco mosaic virus in the late 1800s, thousands of viruses have been identified. Viruses are ubiquitous and are one of the most abundant biological entities.
Viruses include at least two components: (1) genetic material (either DNA or RNA); and (2) a protein coat (the capsid) that surrounds and protects the genetic material. Additionally, in some cases (e.g., in many animal viruses), an envelope of lipids surrounds the protein coat.
Viruses generally enter a host cell in one of two ways. First, the capsid or viral envelope can fuse with the host cell membrane and release the viral material into the host cell. Second, the virus may attach to surface molecules of the host cell, (usually receptors that bind molecules essential to cell's function), and are then passively carried into the cell. In either method, the virus is able to penetrate the cell's membrane and enter the cell. Once inside the cell, the host cell generally breaks down the capsid and viral envelope, thereby exposing the genetic material of the virus. The viral nucleic acid (whether DNA or RNA) then takes over the host cell, using the host cell's own machinery to replicate itself. The virus replicates its genetic material and initiates synthesis of capsid proteins, which spontaneously self-assemble to form the capsid. Non-enveloped viruses are usually then released from the host cell by lysis (the virus causes the lysis of the cell, or the body's own immune system causes lysis). Enveloped viruses are usually released by budding from the host cell and using part of the host cell membrane as their viral envelopes.
As described above, viral mechanisms at the cellular level include cell lysis: the breaking open and subsequent death of the cell. In multicellular organisms, if enough cells die, the whole organism will start to suffer the effects. Although viruses cause disruption of healthy homeostasis, resulting in disease, they may exist relatively harmlessly within an organism. An example would include the ability of the herpes simplex virus, which causes cold sores, to remain in a dormant state within the human body. This is a characteristic of the herpes viruses including Epstein-Barr virus, which causes glandular fever, and varicella zoster virus, which causes chickenpox. Some viruses can cause life-long or chronic infections, where the viruses continue to replicate in the body despite the host's defense mechanisms (common in hepatitis B and hepatitis C virus infections). People chronically infected are known as carriers, as they serve as reservoirs of infectious virus.
Most viral infections of humans and other animals have incubation periods during which the infection causes no signs or symptoms. Incubation periods for viral diseases generally range from a few days to weeks. Somewhat overlapping, but mainly following the incubation period, there is a period of communicability; a time when an infected individual or animal is contagious and can infect another person or animal. This too is known for many viral infections and knowledge the length of both periods is important in the control of outbreaks.
Viruses can be transmitted in many ways. Influenza viruses are spread by coughing and sneezing. The norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the fecal-oral route and are passed from person to person by contact, entering the body in food or water. HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood. Transmission of viruses can be vertical (e.g., from mother to child) or horizontal (e.g., from person to person). Examples of vertical transmission include hepatitis B virus and HIV where a baby is born already infected with the virus. Horizontal transmission is the most common mechanism of spread of viruses in populations. Transmission can be exchange of blood by sexual activity, by mouth by exchange of saliva, or from contaminated food or water, by breathing in viruses in the form of aerosols, and by insect vectors such as mosquitoes.
There are many methods for combating viruses. Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection. However, some viruses (including those causing AIDS and viral hepatitis) evade these immune responses and result in chronic infections. Antibiotics have no effect on viruses, but several antiviral drugs have been developed.
Thus, one line of defense against viruses is the body's immune system, which includes the innate immune system and the adaptive immune system. The innate immune system comprises cells and other mechanisms that defend the host from infection in a non-specific manner. This means that the cells of the innate system recognize and respond to pathogens in a generic way, but unlike the adaptive immune system, it does not confer long-lasting or protective immunity to the host.
When the adaptive immune system of a vertebrate encounters a virus, it produces specific antibodies that bind to the virus and render it non-infectious. This is called humoral immunity. Two types of antibodies are important: IgM and IgG. IgM is highly effective at neutralizing viruses but is only produced by the cells of the immune system for a few weeks. IgG is produced indefinitely.
A second defense of vertebrates against viruses is cell-mediated immunity and involves immune cells known as T cells. The body's cells constantly display short fragments of their proteins on the cell's surface, and if a T cell recognizes a suspicious viral fragment there, the host cell is destroyed by killer T cells and the virus-specific T-cells proliferate.
However, as described above, not all virus infections produce a protective immune response. For example, these persistent viruses evade immune control by sequestration, blockade of antigen presentation, cytokine resistance, evasion of natural killer cell activities, escape from apoptosis, and antigenic shift. Other viruses, called neurotropic viruses, are disseminated by neural spread where the immune system may be unable to reach them. And, HIV evades the immune system by constantly changing the amino acid sequence of the proteins on the surface of the virion.
Vaccination is a cheap and effective way of preventing infections by viruses. Vaccines were used to prevent viral infections long before the discovery of the actual viruses. Vaccines can consist of live-attenuated or killed viruses, or viral proteins (antigens). Live vaccines contain weakened forms of the virus, which do not cause the disease but nonetheless confer immunity. Live vaccines can be dangerous when given to people with a weak immunity, because in these people the weakened virus can cause the original disease. Biotechnology and genetic engineering techniques are used to produce subunit vaccines. These vaccines use only the capsid proteins of the virus. Hepatitis B vaccine is an example of this type of vaccine.
Unfortunately, vaccinations may not be useful once a person has already been infected. And it is not realistically possible to immunize individuals against all viruses, due to the sheer number of viruses, the costs that would entail, any risks of contraction of disease, and the fact that there are some viruses for which there is no vaccine.
And so, anti-viral drugs may be used to treat viral infection. Antiviral drugs are often nucleoside analogues, (simulated DNA building blocks), which viruses mistakenly incorporate into their genomes during replication. The life-cycle of the virus is then halted because the newly synthesized DNA is inactive. This is because these analogues lack the hydroxyl groups, which, along with phosphorus atoms, link together to form the strong “backbone” of the DNA molecule. Examples of nucleoside analogues are aciclovir for Herpes simplex virus infections and lamivudine for HIV and Hepatitis B virus infections. Other antiviral drugs in use target different stages of the viral life cycle. HIV is dependent on a proteolytic enzyme called the HIV-1 protease for it to become fully infectious. There is a large class of drugs called protease inhibitors that inactivate this enzyme.
However, because viruses use vital metabolic pathways within host cells to replicate, they are difficult to eliminate without using drugs that cause toxic effects to host cells in general. And so, unfortunately, there are drawbacks associated with all the above methods for combating viral infection.